Instructions
Download the "GMod" Wire Mod from Wiremod, Mod DB or garrysmod.org.
Unpack the downloaded files and copy them into your "Garry's Mod" installation directory.
Load "Garry's Mod" and join or create a server.
Open the Creation menu and scroll down the tools list. You will find "New Wire Additions."
Select an activation method, such as a button, switch, pressure plate, valve or numpad, and choose the "Off" and "On" value for the device, then place it wherever you desire.
Select a contraption to activate it, such as a thruster, radio, vehicle, hydraulic or turret, and place it wherever you desire.
Open the creation menu again and select the Wire tool. Connect your activation device to your mechanical device using the wire.
Press your activation device and it will give the "On" value you selected in Step 5 to the contraption, making it activate.